Ammonium thiocyanate, also known as ammonium rhodanide, suffers from a caking problem which has been known in the art for a long time. For example, German patent application 2,452,078 from 1974 relates to a process for the reduction of caking in rhodanide products including ammonium thiocyanate. This publication groups ammonium rhodanide, alkali metal rhodanides, alkaline earth metal rhodanides and quaternary ammonium rhodanides together as a single class of materials with regard to the problem of caking.
As a solution to the caking problem for these rhodanides is proposed the addition of a mixture of sodium sulfate and silicon dioxide to the rhodanide. In the examples it is demonstrated that the caking of sodium rhodanide is reduced by addition of a mixture of 1.0 weight percent sodium sulfate and 1.0 weight percent silicon dioxide. However, no further examples with other rhodanides are given in this publication.
An important disadvantage of the solution proposed in this German patent application is that it is generally undesirable to incorporate two weight percent of anti-caking agent in such products since this results in less product per pound for the customer and it adds a significant impurity which can cause problems in the application of such products.
From the article, "How Trace Additives Inhibit the Caking of Inorganic Salts," Phoenix, L., British Chemical Engineering, Vol. 11, No. 1, pp. 34-38 (1966), it can be seen that anti-caking is an empirical art such that one cannot predict from a successful test with one compound that a particular anti-caking agent will work with other compounds. In particular, this publication tests the anti-caking agent ammonia triacetamide [N(CH.sub.2 CONH.sub.3).sub.2 ] for its anti-caking effect on sodium chloride, sodium bromide, potassium chloride and ammonium chloride, among others. The anti-caking effects of ammonia triacetamide were very good, good, none and none, respectively, in these four tests.
More recently, an attempt has been made to determine the relationship between anti-caking effectiveness and crystal growth in the article, "Selection of Anti-Caking Agents Through Crystallization," Chen, Y. L. et al., Powder Technology, Vol. 77, pp. 1-6 (1993). From the article it is apparent that crystal growth is, in some way, related to caking.
However, the conclusion of this article was limited to the statement that performing experiments on crystal growth inhibition gives a better indication of anti-caking effectiveness than caking tests and no clear relationship between the two phenomena was formulated. Thus, even in 1993 anti-caking remains an empirical art where trial and error plays an important role in the search for effective anti-caking agents.
Fatty amines are known to be anti-caking agents from Anonymous Research Disclosure 1980, 189, 31 (England) wherein fatty amines such as tallow amine and stearyl amine are employed as anti-caking agents for pulverent substances such as fertilizers. This publication also discloses a number of other, substituted fatty amines such as hydroxylated and alkoxylated amines, also for use as anti-caking agents for pulverent substances. However, ammonium thiocyanates are not men- tioned in this publication.
Further, East German Patent DD 280,093 discloses the use of fatty acids or fatty amines as an anti-caking coating for ammonium nitrate. A C.sub.12 -C.sub.20 fatty amine is also exemplified in this patent. Soviet Union patent publication 1,650,648 discloses an anti-caking agent for potassium chloride which contains salts of higher fatty amines in admixture with one or more fatty acids.
Finally, European Patent Application 0 048 226 discloses an anticaking agent which comprises inorganic powder, wax and mixtures of cationic amines and fatty acids. This anti-caking agent may be used in fertilizers. The amines mentioned in the text are fatty amines having at least one C.sub.12 or longer alkyl chain.
None of these publications teach or suggest the present invention. Further, in view of the empirical nature of anti-caking, one cannot predict that these amine anti-caking agents will work for ammonium thiocyanate from the information provided in these publications.
The present invention is directed to solving the problem of caking of ammonium thiocyanates. In this regard, the present inventors have sought an anti-caking agent which provides effective anti-caking when added to ammonium thiocyanate in low concentrations. In this manner, the cost of the product is not negatively impacted by addition of the anti-caking agent, nor is there a significant impurity introduced into the ammonium thiocyanate product.
These and other objects of the invention will be apparent from the summary and detailed description of the invention which follow.